Instrument Characteristics

Imaging Performance

The VISIR performance estimates listed on this page are based on observations of mid-infrared calibration standard stars during the re-commissioning of VISIR. As observations are ubtained under a wider range of weather conditions, these post-upgrade sensitivities will be revised at the end of ESO period 95. Median sensitivities are listed as 10 sigma detection limit on a point-source for 1 hour of integration time (median) and 1 hour of execution time including overheads (effective), The median sensitivities quoted are the reference for classification of VISIR service mode observations. A graphical compilation of all sensitivity measurements is available for the N-band and Q-band imaging filters in comparison with theoretical expectations. The filters are linked to their transmission profiles measured at operating temperatures in ascii format.

Spectroscopic Performance

The sensitivities given below are estimates from observations of mid-infrared calibration standard stars. Chopping and nodding along the slit are assumed. The approximate limiting surface brightness for high resolution line observations is around 3*1016W/m2/arcsec2. Graphical representations of the measured sensitivities in comparison with theoretical limits are available for different settings. Note that medium resolution spectroscopy in the N and Q band is offered in visitor mode for all central wavelength settings. See the user manuals for detailed lists of offered settings in service mode.

Detectors

The VISIR imager and spectrometer are each equipped with a 1k x 1k Raytheon Aquarius IBC detector. These detectors have a switchable pixel ("well") capacity. The detectors are used in large capacity mode (6.0 x 106 e- ) for broad-band imaging and in small capacity mode (0.6 x 106 e-) for narrow band imaging and spectroscopy. To avoid saturation due to the high MIR sky background, detector integration times (DITs) are a few milli–seconds in broad-band imaging and may increase up to 2 s in high resolution spectroscopy. Detectors are read from the center outwards. To avoid artifacts due to this new read-out structure, it is recommended that targets are placed a few arcseconds in y above or below the central position of the detector.